while using some 285's to fill. Would a reflector work better than the 285's?

No common answer possible: you have to try and decide.

I have been thining of adding another strobe but have been "dwelling" on which one for fill. I would like to stop some action in the studio so am intrigued with the BXRI250 flash durration. What I don't know is if I use that one with my 500's slower flash durration am I negating the gain with the 250? In otherwords what happens when you mixk flash durrations for main and fill light. Backdrop should not matter as much I beleive.

The slowest flash in the set determines the grade of motion blur, even if it lights "just" the BG. I'd rather get another 250 for it's significantly shorter duration.

> Can I stop enough action like tossing a bolleyball to someone with the 500's duration?

That's borderline. Even the shorter 1/2700 (t 0.5 at full power) of the BRX 250 may be too slow when you consider that the real action stopping flash duration is measured at t 0.1. (Roughly 3 times longer = 1/850s.) Depends of course a lot on how meticulous you are in judging the result...

This is also a matter of the relative orientation of the movement: worst case = fastest if it's lateral to the camera axis, slower if it's along the axis.